In an effort to standardize specifications of drilling muds, the American Petroleum Institute has promulgated a series of test procedures and specified test apparatus that are utilized for the purpose of determining the various characteristics of drilling muds. Among these is a mud filtration device that is used for the purpose of providing a fluid loss standard or index for mud. Basically, two tests are used to determine the fluid loss index.
One test is run at room temperature with about 100 psi pressure differential across the filter of the apparatus. The volume of filtrate flowing from the mud through the filter is measured over a predetermined period of time to provide a fluid loss number.
A second test is similar, but run at higher pressure and higher temperature with the pressure differential being approximately 500 psi and the temperature being at the anticipated bottom hole temperature, usually between 250.degree. F. and 400.degree. F. Again, the filtrate from the mud passing through the filter is measured over a predetermined period of time to provide the fluid loss number. In the low temperature apparatus a 7 square inch filter is disposed horizontally at the bottom of the test or pressure vessel through which the filtrate passes, while in the high temperature apparatus a 31/2 square inch horizontally disposed filter is used. To correlate the tests, the number derived from the high temperature test is doubled.
The tests have proved to be adequate for the standardization of a fluid loss number or index which provides a comparison between various drilling muds. Recently, the composition of the drilling muds has changed to include new materials that have become available. The new muds are generally referred to as high temperature polymer drilling fluids. The apparent advantage of such muds is that the materials used therein can be standardized. Also, the number of materials used in the mud can be reduced. A discussion of such muds and the testing thereof is available in a paper entitled "The Development of a Deflocculated Polymer Mud for HTHP Drilling" by J. C. Abdon, B. L. Jackson, and G. S. McClellan. The paper was published in 1989 by the Society of Petroleum Engineers under S.P.E. number 17924. The new polymer drilling muds utilize as a viscosifier, a polymer known as xanthan gum. Filtration control is provided in the mud by a polyanionic lignin polymer and deflocculation is provided by an acrylic co-polymer. Weighting is provided by the use of barite.
It was noticed during the standard A.P.I. filtration testing of the polymer muds at the higher temperature that the fluid loss number seemed to vary substantially and it was further noticed that the fluid loss number did not seem to be indicative of what was actually happening in the well. An analysis of the problem, upon recognizing the foregoing contradiction, seemed to indicate that at temperatures above 250.degree. F., and in the presence of oxygen, the xanthan gum oxidized, causing the mud viscosity to drop and permitting weighting materials in the mud to settle out of suspension. Weighting materials may constitute as much as 48 percent by volume of the mud. Such a phenomena appeared to occur in the test cell where oxygen was present, but not in the well where at bottom hole, no oxygen was available for the oxidation process. From this realization, it was determined that the inconsistent results being obtained in the filtration test unit of the American Petroleum Institute standard resulted from the settling out of the weighting materials, such as barite, onto the horizontally positioned filter located at the bottom of the test vessel.
The resulting loss of viscosity promoted the settling of barite. Two phases developed: a barite/liquid phase on the bottom and a liquid/colloid phase above. This upper phase contained a predominant portion of the lignin polymer. When filtration was initiated, the colloids (including the lignin polymer) were filtered out in the upper portion of the barite plug. The result was a limited concentration of lignin polymer in the filter cake and hence a high rate of filtration.
Some of the apparatus and methods utilized for the testing and determination of drilling mud characteristics are contained in U.S. Pat. No. 3,055,208 issued Sept. 25, 1962 to J. P. Gallus; U.S. Pat. No. 3,289,467 issued Dec. 6, 1966 to H. W. Parker et al.; and U.S. Pat. No. 4,538,452 issued Sept. 3, 1985 to Ivan Hrvojic. While none of the foregoing patents illustrate apparatus that is precisely like that proposed herein, various characteristics of the systems discussed are shown in the patents. For example, in Parker U.S. Pat. No. 3,289,467, a vertical filtration system is utilized for determining drilling mud characteristics such as water loss under dynamic conditions.
An object of this invention is to provide improved polymer mud testing apparatus and testing methods that correlate with the long-standing fluid loss standards developed over the years by the American Petroleum Institute.